This invention relates to interferometers and more particularly to the stepping of the optical path difference in an interferometer.
Interferometers are instruments which can be used to measure a linear distance with great accuracy or, to measure the wavelength of radiation with great accuracy. The Michaelson interferometer is a well known type of interferometer which splits the incoming light beam into two beam portions by means of a semi-reflecting surface so that the two portions can travel over two different paths. It then recombines the two beam portions after they have been reflected by separate plane mirrors to produce fringes. Proper operation of the basic Michaelson interferometer requires that the alignment of the plane mirrors be maintained to within approximately a second of arc. This alignment is difficult to maintain when one of the mirrors is moved along the normal to the mirror surface, which is the case in most interferometers.
An improved interferometer utilizes cube-corner reflectors in place of the plane mirrors in order to return the beams in a direction parallel to their directions of incidence. Such devices are referred to as retro-reflectors. The cube-corner retro-reflector returns the beam in precisely the same direction as the incoming beam in spite of slight angular misalignments. However, difficulty is encountered in producing cube-corner retro-reflectors because the three perpendicular plane mirrors comprising the cube-corner must be perpendicular within a second of arc.
Another type of retro-reflector used in interferometers is a "cat's eye" retro-reflector. The cat's eye consists of a parabolic primary mirror and a flat secondary mirror positioned at the focus of the primary mirror. The cat's eye retro-reflector has the advantage of the cube reflector in that it is tolerant to moderate misalignment and has the further advantage that it is much easier to manufacture.
With the advent of cat's eye retro-reflectors, the use of interferometers under non-laboratory conditions became possible. However, in non-laboratory environments, the interferometer is subject to vibrations that could seriously degrade its performance. The affects of low-frequency vibrations can be compensated for by a servosystem which moves one of the cat's eye retro-reflectors or corner-cube retro-reflectors back and forth so as to maintain a predetermined optical path difference. This predetermined optical path difference is charted by a separate visible monochromatic light beam such as is generated by a laser. The laser beam produces interference fringes having amplitudes that vary sinusoidally as one of the retro-reflectors is moved. The servo nulls (produces no error correcting signal) on a particular portion of the sinusoid, for instance the trough. Such an interferometer is described in an article printed in Applied Optics, Volume 9, page 301 on February, 1970.
The prior art servosystem for changing the optical path difference between the two cat's eye retro-reflectors, as described, for example, in U.S. Pat. No. 3,535,024, take the form of a dual-mode control system. This type of control system steps the optical path difference from one null position to another in the open loop mode and utilizes the interference fringes generated by the reference light beam as the error signal in the closed loop mode for nulling the movement of the retro-reflector as it approaches the next fringe area. Thus, as explained in U.S. Pat. No. 3,535,024, the optical path difference is stepped along by the use of open-loop signals which nudge the mirror mechanism to the next fringe area, the voltage utilized for this nudging action being sufficient to move the retro-reflector a certain distance to the next null area. Upon anticipating the approach of the null area, a closed-loop error voltage is generated by the reference interference pattern. This error voltage is utilized to maintain that null position during the time a reading is taken.
This type of interferometer control mechanism is not satisfactory for the purpose of stepping the optical path difference of a fourier interference spectrometer at the high speeds required by certain applications. In addition, this type of prior art servo control inherently limits the internal modulation that may be utilized to 1/2 the wavelength of the reference light source, thereby severely limiting the interferometer's usefulness in the analysis of certain radiation.
Internal modulation in interferometers is a well known method of modulating the radiation to be analyzed. The optical path difference in the interferometer is modulated by an amount which, optimally, equals one-half the wavelength of the radiation to be analyzed. After detection, the modulated signal is phasesensitively demodulated before further signal processing. This process reduces the effects of atmospheric and 1/f noise.